1. Field of the Invention
The invention relates in general to integrated circuit oscillators, and more particularly, to digitally controlled oscillators (DCOs).
2. Description of the Related Art
Ring oscillators are important building blocks in many digital communications systems. For example, they are used in applications such as clock recovery circuits for serial data circuits communications, disk-drive read channels, on-chip clock distribution and integrated frequency synthesizers. A ring oscillator in its simplest form consists of an odd number of inverters connected in a circular chain. The circuit oscillates since it has no stable operation point. Ring oscillator frequency is determined by propagation time through the chain of inverters.
The oscillation frequency of a voltage-controlled ring oscillator (VCO) typically is monitored and adjusted based upon an analog voltage. However, such prior VCOs often have been especially sensitive to control noise or power supply noise. Moreover, integration of both analog and digital components in an integrated circuit can be especially challenging.
A digitally controlled oscillator (DCO) is one alternative to a VCO. One implementation of a DCO employs digitally controlled capacitance inserted in a ring oscillator to select oscillation frequency. Capacitors of different sizes can be digitally switched in and out of a ring oscillator inverter chain to control delay, and therefore, the oscillation frequency.
There have been problems with implementing DCOs, however. For example differences in capacitance applied to different inverters in an inverter chain can limit oscillator frequency range. Parasitic capacitance also can limit frequency range. Power supply noise and ground noise can impact frequency stability. Frequency glitches may occur during adjustment of oscillator frequency due to different switching delays for different capacitors, for instance.
Thus, there has been a need for improvement in DCOs using digitally controlled capacitors. The present invention meets this need.